1. Field of the Invention
The present invention relates to multimode planar spectrographs and methods for their fabrication, and more particularly to planar spectrographs for demultiplexing coarse wavelength division multiplexed (CWDM) optical signals, and methods for fabrication of such spectrographs employing silicon-based processing.
2. Description of the Related Art
Emerging applications of wavelength-division-multiplexing (WDM) in local-area networks require wavelength demultiplexers that are compact, low-cost, manufacturable in high volumes, and most of all compatible with multimode fiber input. Multimode operation precludes the many single-mode waveguide demultiplexers used in dense WDM systems for telecommunications purposes.
A planar spectrograph demultiplexer is a two-dimensional grating spectrometer comprised of a slab waveguide which confines the light in the vertical direction and a concave reflection grating which simultaneously diffracts and images the input light, separating different wavelengths in an output image plane.
Referring to FIG. 1, formation of a planar spectrograph 10 within a layered dielectric slab waveguide for single-mode fiber applications is illustratively shown. Spectrograph 10 is formed by depositing a plurality of glass layers 17 to form slab waveguide 16 having a numerical aperture and a thickness of a center core layer 17b closely matched to single-mode fiber. Upper and lower cladding layers 17a and 17c are also provided. In FIG. 1, a concave trench 12 is lithographically patterned and then etched into the slab waveguide 16 with a grating structure 14 formed on a near face. The grating facets are then metallized by access through the open trench 12.
To enable the highest performance within the smallest possible footprint of spectrograph 10, aberrations in the diffracted images are reduced by means of an acircular grating curve with variable grating facet pitch (i.e., chirp). Manipulating the two degrees of grating freedom, curvature and pitch, allows for two stigmatic wavelengths. Grating resolution is optimized by varying the two stigmatic wavelengths to minimize aberrations over the full wavelength range of operation. The grating theory and design procedure have been developed along the lines described in the art.
In addition, for maximum diffraction efficiency, grating facets 28 are blazed (i.e., angled) to spectrally reflect the incident light to the diffracted image of a wavelength (the blaze wavelength) central to the wavelength range of operation. A saw-toothed echelette grating facet 28 may be employed. This optimum blaze angle will vary with facet position along the curved grating.
By allowing for arbitrary lateral profiles, the lithographic patterning and etching of grating 14 in spectrograph 10 facilitate the realization of such acircular, chirped gratings having continuously variable blazing.
For local area network (LAN) WDM applications, a critical demultiplexer feature is compatibility with input from multimode fiber (MMF). In particular, this would require a thick core layer (e.g., xe2x89xa762.5 microns) for the spectrograph slab waveguide 16. Depositing and etching glass layers 17 to form a MMF-compatible planar spectrograph in a manner similar to that for the single-mode device are no longer practically or technically feasible for glass slab waveguide layers of these thicknesses. Past efforts at realizing MMF-compatible spectrographs fabricated thick-core glass slab waveguides by stacking thin glass sheets or by performing ion-exchange. Gratings were formed on an end face of the slab waveguide by replication of a ruled master grating, by holography, or by epoxying a separately ruled or etched grating. These approaches suffer from several drawbacks and difficulties with respect to their ultimate levels of performance and their ability to be packaged.
Referring to FIG. 2, a multimode planar spectrograph 29 representative of the prior art is illustrated where a reflective grating 24 is affixed to a slab waveguide glass stack 22 on an end that has been ground and polished into a concave surface. Stack 22 includes cladding layers 21 and a core layer 19. Grating 24 is pre-processed to form an echelette grating 26 before being attached to slab waveguide 22. Fabrication of an optimized chirped, acircular grating complete with continuously variable blazing similar to that for the single-mode spectrograph is non-trivial by traditional means. Control over one of the grating design parameters, chirp or curvature, must often be sacrificed, affording the possibility for only one stigmatic wavelength versus two for the grating patterned lithographically in spectrograph 10 (FIG. 1). Grating 24 will typically be formed through a grating ruling process or a holographic printing process. Grating ruling employs mechanical scribing with a diamond tip to create facets 28. To chirp the grating and/or vary the blazing, it is necessary to adjust the diamond tip between scribes. Since continual adjustment between successive scribes would be laborious, time-consuming, and expensive, the grating pitch and blaze are normally changed step-wise for only a few segments of the length of a ruled grating. Holographic grating definition can provide continuous chirping for aberration correction but with only one resultant stigmatic wavelength. Furthermore, blazing of holographic grating facets is limited.
Since grating 24, input fiber 27, and output array 31 are attached to slab waveguide 22 in separate steps, costly low throughput active alignment of either the grating or the input/output elements is required when using a corrected grating. Relative misalignments may occur and are an additional source of concern for aberrations in the output images.
By means of the LIGA process, multimode planar spectrographs have been made in polymer material systems using deep-etch X-ray lithography with parallel synchrotron radiation. Polymers, however, are generally frowned upon for use in LAN datacom transceivers due to their uptake of moisture in non-hermetic packaging, high sensitivity to operating temperature changes, and/or inability to survive elevated temperatures during transceiver solder reflow processing.
Meanwhile, grating spectrometers constitute a platform that will scale to higher WDM channel counts and tighter channel spacings with less added cost and more consistent performance than will presently competing serial-processing multimode demultiplexers that use dielectric interference filters.
Therefore, a need exists for multimode spectrographs, and a method for fabrication thereof, which are inexpensive, easy to manufacture, environmentally rugged, and unrestricted in terms of their grating design.
The present invention provides for multimode-fiber compatible spectrographs which are fabricated employing planar, batch processing of silicon wafers. By means of silicon deep reactive-ion etching, high-quality, aberration-corrected gratings defined lithographically and capable of realizing minimum device dimensions are etched into a silicon substrate. The diffraction grating is integrally formed in the substrate so as to be in operative relationship with input light to diffract and image the wavelength components of the input light to an output detector, fiber, or integrated waveguide array. Input/output coupling and passive alignment features can be integrated directly into the silicon substrate to facilitate low-cost, high-volume packaging. The thick-core slab waveguide responsible for coupling input light to and diffracted light from the etched grating may be formed in a plurality of ways, for example, as the top silicon layer in a thick-film silicon-on-insulator (SOI) wafer or as a hybrid thin glass element dimensioned and configured to fit within a recess formed within the silicon substrate.
A planar spectrograph for demultiplexing optical wavelength signals of the present invention includes a monolithic substrate. The substrate includes a diffraction grating etched therein, and the diffraction grating is integrally formed in the substrate to be in operative relationship with input light to diffract and reflect the input light to a detector. A recess is formed in the substrate, and a slab waveguide is dimensioned and configured to fit within the recess. The waveguide guides the input light to and from the diffraction grating.
In other embodiments, the substrate preferably includes silicon. The substrate defines a plane and the diffraction grating preferably includes a thickness perpendicular to the plane which is greater than or equal to a core diameter of multimode fiber. The substrate may include a feature integrally formed therein to accept an optical fiber. The substrate may include a feature integrally formed therein to accept a lens in operative relationship with the optical fiber. The optical fiber may include a single mode fiber or a multimode fiber. The slab waveguide may include glass. The diffraction grating may include an echelette grating profile. The diffraction grating may include a metallized grating to promote reflection. An index matching fluid is preferably disposed in a gap formed between the diffraction grating and the slab waveguide. The detector may include at least one of output fibers, photodetectors and a waveguide array. The detector may include an array of photosensitive panels which are selectively activated when the light diffracted from the diffraction grating falls thereon.
A planar spectrograph for demultiplexing optical wavelength signals in accordance with the present invention includes a silicon-on-insulator (SOI) structure including a monolithic first silicon layer forming a slab waveguide core, a second substrate layer and a waveguide cladding layer disposed therebetween. The SOI substrate includes a diffraction grating etched in the first silicon layer of silicon down to the waveguide cladding layer. The diffraction grating is integrally formed in the first silicon layer to be in operative relationship with an input light source to diffract and reflect input light to a detector.
In other embodiments, a thickness of the first silicon layer is greater than or equal to a core diameter of multimode fiber. The SOI substrate may include a feature integrally formed therein to accept an optical fiber. The SOI substrate may include a feature integrally formed therein to accept a lens in operative relationship with an optical fiber. The input light source may include one of a single mode fiber and a multimode fiber. The diffraction grating may include an echelette grating profile. The diffraction grating may include a metallized grating to promote reflection. The spectrograph may include a detector positioned to receive light diffracted from the diffraction grating. The detector may include an array of photosensitive panels which are selectively activated when the light diffracted from the diffraction grating falls thereon. The detector may include at least one of output fibers, photodetectors and a waveguide array.
A method for fabricating a planar spectrograph includes the steps of providing a monolithic substrate, lithographically patterning the substrate, deep etching the substrate in accordance with the lithographic pattern to integrally form a diffraction grating and a recess in the substrate, and securing a slab waveguide in the recess for directing light onto the diffraction grating and directing diffracted light from the diffraction grating.
In other methods, the deep etching preferably includes etching by reactive ion etching. The deep etching is preferably performed to a depth equal to or greater than a core diameter of a multimode fiber. The method may include the step of coating the diffraction grating to promote reflection. The method may include the step of applying an index matching fluid between the diffraction grating and the slab waveguide. The method may include the step of forming a groove in the substrate for an optical fiber. The method may include the step of forming a socket in the substrate for a lens.
Another method for fabricating a planar spectrograph includes the steps of providing a silicon-on-insulator substrate, lithographically patterning a first silicon layer of the substrate, and deep etching the first silicon layer in accordance with the lithographic pattern to integrally form a diffraction grating in the first silicon layer.
In other methods, the deep etching preferably includes reactive ion etching. The deep etching is preferably performed to a depth equal to or greater than a core diameter of multimode fiber. The method may include the step of coating the diffraction grating to promote reflections. The method may include the step of coating edge surfaces of the first silicon layer with an anti-reflection coating (ARC). The method may include the step of forming a groove in the silicon-on-insulator substrate for an optical fiber. The method may include the step of forming a socket in the silicon-on-insulator substrate for a lens.